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Comet Shoemaker-Levy 9 (formally designated D/1993 F2) was so-named because it was the ninth short-period comet discovered by Carolyn and Eugene M. Shoemaker and David Levy. It was first detected in a photograph taken on the night of March 24, 1993 with the 0.4-meter Schmidt telescope at the Mount Palomar Observatory in California, and subsequently observed by many other astronomers. It was the first comet observed while it was orbiting a planet (http://www.physics.sfasu.edu/astro/sl9/cometfaq.html#Q2.4) rather than the Sun. The comet was also unusual because it was in fragments (ranging in size up to 2km in diameter), evidently due to a close encounter with the planet Jupiter in July 1992. It is thought to have been pulled apart by tidal forces when it approached closer than its Roche limit.
During the period July 16 – July 22 1994, over twenty fragments of the comet collided with Jupiter's southern hemisphere at 60 kilometers (37 miles) per second, providing the first direct observation of the collision of two solar system objects. The collision resulted in disruptions in Jupiter's atmosphere, such as plumes and bubbles of gas, and dark spots in the atmosphere which remained visible for several days.
The event was closely observed and recorded by astronomers worldwide, because of its tremendous scientific importance. The event also had a large amount of coverage in the popular media (unusual for an astronomical event), probably in part due to the fascination with the possibility of comet impacts with Earth.
The largest impact was caused by Fragment G, at 7:32am (UTC) on 18 July. This released energy estimated to be the equivalent of six million megatons of TNT (the total estimated for all the impacts is forty million megatons). The mark left as a result of the impact was greater than the diameter of the Earth.
The event brought to the fore Jupiter's role in sweeping the inner solar system free of debris, which is thought to be a prerequisite for unbroken development of life. It also served as a dramatic example of the role played by the Interplanetary Superhighway in solar system dynamics.
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